The invention is directed to a nanocrystal production method.
The history of synthesizing nanowires made of compounds that are difficult to melt goes back to the 1970s, when it became known that as the cross-section of whiskers decreased their strength limit attained theoretical values. The practical utility of the whiskers became apparent when producing composite materials, high-strength and highly modular materials based on polymers, metals, ceramics, and graphite for novel materials. Whiskers made of silicon carbide and silicon nitride, aluminum oxide and aluminum nitride, etc., were mass-produced; they were studied not only due to their strength, but also from a functional standpoint—heat conductivity, piezoelectricity, electrocondensation, etc. Whiskers were grown from a solid batch of the basic element, to which were added compounds of the appropriate element that, when heated and at nearly atmospheric pressure (1.2 atm), easily convert in a reactive or inert milieu to the gaseous phase. Analysis of the dimensions obtained indicated that the dispersity of the whiskers in principle is a function of the size of the drop of the basic element on the surface of the seed crystal, whose growth is stimulated by the VLS (vapor-liquid-solid) mechanism and is practically impossible to control. The dimensions of most of the whiskers was in a range from 1-500 μm in diameter, with the vast majority having a diameter of 100-300 μm. Attempts to produce nanowhiskers at normal air pressure failed. Piezoelectric whiskers made of aluminum nitride with a cubic modification (patent 2016 298 RF SO 1 V 21/072) that were grown from the gaseous phase at 1100-1300° C. in a nitrogen or ammonia atmosphere from an Al+AlF3 batch (patent 2136 587 RF SO 1 V 21/072 BI 1999 No. 25) aroused the greatest interest from a functional point of view. But keeping the diameter of the whiskers obtained in this manner below 0.15 μm proved not to be possible, and thus they made up just 1.5-2% by volume of the total product mass and their separation increased the cost of the whiskers 1000-fold. The attempt to grow AN nanowhiskers from the gas phase by depositing aluminum hydride on a hot substrate in a nitrogen atmosphere (Nanostruct. Mater. 1992, Vol. 4, p. 269) was responsible for only an insignificant increase in the yield of whiskers (less than 0.1-0.2% by volume) with a minimum diameter of 1000 Å.
Initial experiences with mass-produced whiskers made from aluminum nitride (Redkino chemicals factory, Moscow District) date back to the year 1975. They were used as heat insulation of the hot portion of a pressure chamber working under a nitrogen pressure of 200 MPa and at a temperature of 2000° C. We discovered that whiskers that were in next to the hot zone transformed by recrystallization from large 250 μm sizes to small sizes from 0.1 to 1 μm, and at a rate of 98%. Increasing the nitrogen pressure to 500 MPa at a temperature that remained the same reduced the diameter of the crystal only minimally to 0.1-0.5 μm.
The next experiment attempted to grow the whiskers from a batch comprising aluminum powder, with an aluminum hydride added, at a nitrogen pressure of 1000 MPa and a temperature of 2300 K. Of the crystals now obtained, 99% had a diameter of 10-70 Å. After impregnation with a gas mixture of He+30% O2, there was high absorbability with respect to air with a portion of radioactive isotopes of xenon, cesium, etc.
The present invention is an improvement over the foregoing.